In such a channel, two principle impairments are observed:
1) multipath fading which may be fast or slow depending on the movement of the receiver, and its position with respect to the transmitter and other items which might affect reception and 2) intersymbol interference ("ISI") due to delay spread. A variety of techniques, such as 1) coding, 2) equalization and 3) antenna diversity have been employed to address these problems with varying degrees of success, See, for example, Lee-Fang Wei, "Coded M-DPSK with Built-In Time Diversity for Fading Channels", IEEE Transactions on Information Theory, Vol. 39 No. 6, pp. 1820-1839, November 1993; and Jack H. Winters, Jack Salz, Richard D. Gitlin "The Impact of Antenna Diversity on the Capacity of Wireless Communication Systems", IEEE Transactions on Communications, Vol. 42 No. 2/3/4, pp. 1740-1751, February/ March/ April 1994. Both of these papers are incorporated by reference herein.
Single techniques have various drawbacks as discussed below. Coding can provide up to a 3 to 4 dB decrease in required average signal energy per information bit received from each antenna per one-sided power spectral density of an additive white Gaussian noise ("AWGN"), E.sub.b /N.sub.o, for a given bit error rate ("BER") with AWGN. With fast fading, more complex codes with interleaving can achieve diversity gain with time diversity sufficient to perhaps permit a 64 kbps rate of data transmission in a 30 kHz channel. See the Wei paper cited above. However, such complex codes add processing expense and delays and, with slow fading, as with stationary users such as mobile users in a stopped vehicle or portable users transmitting from a fixed position such as an office or home, coding alone cannot achieve diversity gain.
Equalization can eliminate ISI, while providing diversity gain when delay spread is present. More complex methods can partially suppress co-channel interference ("CCI") when delay spread is present, offering the possibility of some capacity increase through higher channel reuse.
Antenna arrays can provide diversity gain by receive diversity. Transmit diversity, the use of multiple antennas, cannot alone provide diversity gain, but when the transmit frequency of each antenna is slightly different or offset, slow fading can be converted to fast fading at the receiver. This conversion has advantages when combined with coding. See, "Orthogonal Polarization and Time Varying Offsetting of Signals for Digital Data Transmission or Reception", U.S. Ser. No. 08/159,880 filed Nov. 30, 1993 which is assigned to the assignee of the present invention and which is incorporated by reference herein.
Arrays of transmit antennas can also be employed to obtain up to M independent spatially-isolated channels in the same frequency channel between the base station and mobile. With M antennas, linear combining of signals at the receiver can suppress up to M-1 interferers, permitting M-N+1 simultaneous channels each with the same performance as a single channel with an N-fold diversity gain. See, the Winters et al. paper cited above.
Despite the existence of this known technique, a need still remains for a technique to obtain significantly higher or near maximal data rates with a minimal increase in complexity while addressing a full range of multipath fading from slow to fast.